Surge control for fluid compressors

ABSTRACT

A control means for preventing surging under changing or low flow conditions in a fluid compressor such as an air compressor which includes a rotor and diffuser and at least two stages of compression is disclosed. Compressed air from the n+1 stage of the compressor is injected into the diffuser of the nth or lower stage of the compressor through a collection chamber which is disposed about the diffuser.

nited States atent 1 Siivern et a1.

SURGE CONTROL FOR FLUID COMPRESSORS Inventors: David Harold Silver-n,Los Angeles;

Stanley J. Minton, Woodland Hills, both of Calif.

Assignee: Barodyne, Inc., Los Angeles, Calif.

Filed: Oct. 12, 1971 Appl. No.: 188,199

US. Cl 415/52, 415/11, 415/27, 415/147, 415/DIG. 1 Int. Cl. F0ld 1/12Field of Search 415/11, 27,28, 45, 415/52, 147, DIG. 1

References Cited UNITED STATES PATENTS 11/1953 Klein et a]. 4l5/D1G. 1

[ June 26, 1973 2,599,470 6/1952 Meyer 415/D1G. 1 3,362,629 1/1968Papapanu 4l5/D1G. l

FOREIGN PATENTS OR APPLICATIONS 619,722 3/1949 Great Britain 415/D1G. 1

Primary Examiner-C. J. Husar Att0rneySpensley, Horn and Lubitz [57]ABSTRACT A control means for preventing surging under changing or lowflow conditions in a fluid compressor such as an air compressor whichincludes a rotor and diffuser and at least two stages of compression isdisclosed. Compressed air from the n+1 stage of the compressor isinjected into the diffuser of the nth or lower stage of the compressorthrough a collection chamber which is disposed about the diffuser.

9 Claims, 4 Drawing Figures PAIENIEBmzs I975 SHEEIZUFZ 1/10 b690406741/5? Sim/45rd 44/4/70 INVENTOR.

SURGE CONTROL FOR FLUID COMPRESSORS BACKGROUND OF THE INVENTION 1. Fieldof the Invention The invention relates to-means for controlling surgingin centrifugal compressors.

2. Prior Art Centrifugal compressors which utilize rotors and diffusersto compress fluids such as air are well known in the art. While thesedevices are efficient for a constant predetermined flow they exhibitundesirable characteristics such as surging when the flow drops belowthe predetermined level. For example, in some centrifugal compressorssurging occurs when the flow through the compressor drops below 80percent of the compressors predetermined or optimum flow. The surgingresults from the building up and breaking down of various boundaries andlayers of fluid within the compressor. The surging is an undesirablecharacteristic since it results in an uneven flow of air from thecompressor.

Numerous attempts have been made in the prior art to control surging.One method for controlling surging is to permit the compressor tooperate at its predetermined or optimum flow level independently of thedemand for fluid from the compressor. This is generally accomplished bydischarging the high pressure fluid at the outlet of the compressor whenthe fluid is not being used. An obvious disadvantage to this method isthe fact that the work performed on the fluid in compressing it iswasted when the high pressure fluid is discharged.

Another method for controlling surging is that described in U.S. LettersPatent No. 2,660,366. In this patent compressed fluid such as air isrecirculated through the compressor by injecting it into the stream ofair before that stream contacts the compressor rotor.

It appears from a review of the prior art techniques for preventingsurging that the principal efforts have been directed towards preventingsurging caused by the compressor rotor. Tests have indicated that inmany centrifugal compressors the rotor is more tolerable of low flowconditions than is the diffuser. For example, in some instances it hasbeen found that a reduction of flow of about 20 percent will causesurging in a compressor and that the cause of surging was attributableto the flow of air in the diffuser and not the flow of air in the rotor.On the same compressor it was found that a decrease of flow of about 60percent was required before surging problems attributable to the flow ofair in the rotor could be detected. Thus, the prior art attempts tosolve surging by injecting air upstream of the rotor have been somewhatmisdirected since these attempts are aimed at preventing flow problemsin the rotor area, when in fact the cause of the surging is firstattributable to problems within the diffuser.

SUMMARY OF THE INVENTION A surge controlling means for a centrifugalcompressor having at least two stages, each of which comprise a rotorand diffuser is described. Air from one stage of the compressor isdirected in'to the diffuser of a lower stage of the compressor through acollection chamber disposed about that diffuser. Control means areprovided within aline whichcouples the collection chamber to a higherstage of compression to selectively allow the flow of air from 'thehigher stage to the lower stage when the flow of air to the compressordrops below a predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates one prior arttechnique for preventing surging in a centrifugal compressor; saidcompressor is illustrated in a partial cutaway view;

FIG. 2 is a plan view of a two-stage compressor utilizing the surgecontrol means of the present invention;

FIG. 3 is a partial cutaway view of the portion of the compressor shownin FIG. 2 generally along section line 3-3; and

FIG. 4 is a sectional view of the collection chamber, diffuser and rotorof the compressor illustrated in FIGS. 2 and 3.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, a typicalcentrifugal compressor for compressing a fluid such as air isillustrated along with a means used in the prior art for preventingsurging. The compressor illustrated in FIG. 1 includes a first stage ofcompression 10 and a second stage of compression 20. The air or otherfluid to be compressed enters the compressor through inlet 12, is thencom pressed within the first stage of compression 10, leaves the firststage of compression through outlet 16 and then enters the interstageheat exchanger 17. The air is typically cooled in between stages ofcompression and as illustrated in FIG. 1, the air leaving the heatexchanger 17 enters the second stage inlet 18 where it is compressed bythe second stage of compression 20. The outlet air from outlet 19 of thesecond stage of compression 20 may then be used to perform numeroustasks as is presently done in the art such as driving pneumatic tools ormay be compressed in additional stages of compression.

The first stage of compression 10 includes a rotor 14 and a volutediffuser 15. Likewise the second stage of compression 20 includes arotor 22 and a diffuser 21. Both rotors 14 and 22 are directly connectedto a drive shaft 28 which is rigidly coupled to a pinion gear 27. Thepower for the compressor is obtained from a motor 23 which is coupled topinion gear 27 through bull gear 24, pinion gear 25 and bull gear 26.

The prior art surge control means illustrated in FIG. 1 includes aninlet valve 13, an outlet control valve 30, recirculating control valve31 and a recirculating line 11. The recirculating line 11 interconnectsthe outlet 19 of the second state of compression 20 with the inlet 12 ofthe first stage of compression 10. Valve 11 disposed within therecirculating line 11 controls the amount of outlet air which isrecirculated within the compressor. The inlet valve 13 controls theamount of air which enters the compressor while the outlet valve 30 isused to control the outlet flow of air from the compressor.

Centrifugal compressors such as the one illustrated in FIG. l have ahigh efficiency while operating at a predetermined or optimum flow rate.In many applications a constant demand for air is not required and thusthe flow of air from outlet 19 is not a constant. For example, where theair from the compressor is utilized to drive a plurality of pneumatictools, the demand for air will be a function of the number of tools inoperation and will be a variable. As previously explained, when the flowthrough the compressor is decreased below a predetermined value surgingoccurs. In the prior art method for preventing surging illustrated inFIG. 1, air from the outlet 19 is fed into the compressor at the inletto the first stage of compression 10. Typically control means sense thedemand for air and as the demand decreases, valve 30 prevents all theair compressed by compressor stage 20 from leaving the compressor.Simultaneously valve 31 opens, allowing some of its air to enter theinlet of compressor stage through line 11. Also the inlet air tocompressor stage 10 is throttled by valve 13.

The surge control means illustrated in FIG. 1 has the disadvantage ofwasting the energy utilized to compress the air in compressor stages 10and 20. The high pressure air from outlet 19 when recirculated into theinlet 12, expands and is again recompressed within the compressor.Additionally, as previously explained, the injection of air by line 11into the rotor 14 is intended to present the compressor surgingassociated with the rotor. Tests have indicated that in a typicalcentrifugal compressor the rotor is more tolerable to low flowconditions than is the diffuser. Thus, the prior art surge controltechnique illustrated in FIG. 1 has not provided a satisfactory surgecontrol.

Referring to FIGS. 2' and 3, a compressor of substantially the sameconstruction as the compressor of FIG. 1 is again illustrated but with asurge control means built in accordance with the present invention. Thecompressor comprises a first stage of compression 40 and a second stageof compression 50. Inlet air to the compressor enters through inlet 44,is then compressed by the first stage of compression 40, and is thendelivered to the inlet 46 of the second stage of compression throughline 45. Typically, a heat exchanger is coupled into line 45 so that theheat of compression from the first compression state 40 may be removedbefore the air is recompressed within the second stage of compression50.

The first stage of compression includes a rotor 41 and a volute diffuserchamber 42 defined by manifold 52. The second stage of compressionlikewise includes a rotor and diffuser 43 which for the sake of clarityhave not been illustrated in detail in FIGS. 2 or 3. The second stage ofcompression may be similar to the first stage of compression 40.

The driving means for the compressor includes a motor 64 which isdirectly connected to a bull gear 65. Bull gear 65 cooperatively engagesand drives a pinion gear 66 which is axially mounted on a common shaftwith a bull gear 67. Bull gear 67 cooperatively engages a pinion gear 68which in turn drives drive shaft 69. The drive shaft 69 is coupleddirectly to the rotor 41 of the first state of compression 40 and mayalso be coupled to the rotor of the second stage of compression 50.

The compressor and in particular the rotor and diffuser may be builtutilizing standard prior art techniques. While in FIGS. 2 through 4 thesurge control means of the present invention is described in conjunctionwith a volute diffuser such as the one defined by manifold 52, it willbe obvious to one skilled in the art that other types of diffusers suchas a spiral diffuser may be utilized with the presently disclosed surgecontrol means.

Referring to FIGS. 2, 3 and 4, a collection chamber 70 defined by amanifold 71 is axially disposed about one side of diffuser manifold 52.As best illustrated in FIG. 3, the collection chamber 70 is partlydefined by the outer surface of manifold 52 and by manifold 71.

Manifold 71 abuts an inner radius of manifold 52. Orifices 63 disposedthrough manifold 52 allow the collection chamber to communicate with thediffuser chamber 42. Referring particularly to FIG. 4, the orifices 63are generally tangentially disposed through manifold 52 such that theair entering the diffuser chamber 42 will be injected into that chamberin the same direction as the direction of air forced into the chamber byrotor 41; that is, the air is injected into the chamber in the directionof the prevailing flow.

An inlet 54 formed by a portion of the collection chamber manifold 71allows the inlet 54 to communicate with collection chamber 70. Thediffuser return line 60 interconnects the outlet 62 of the second stageofcompression 50 with the inlet 54 of the collection chamber 70. A valve61 which is disposed within line 60 is used to throttle the air or otherfluid which will flow from outlet 62 to the inlet 54.

The collection chamber 70 defined by manifold 71 with its inlet 54 maybe an ordinary metal part manufactured utilizing known techniques. Theline 60 and valve 61 may be standard components known and used inconjunction with fluid compressors.

When the compressor illustrated in FIGS. 2 through 4 is operating at itsoptimum or predetermined flow the valve 61 disposed within line 60 maybe completely closed so that no air will flow from outlet 62 into inlet54. As the flow from outlet 62 decreases below its optimum or apredetermined value, valve 61 opens, allowing air from the outlet 62 toflow into the collection chamber 70. The amount of air which is allowedto flow through line 60 is regulated by valve 61, and as the demand forair from the compressor is decreased more air is allowed to flow intothe collection chamber 70. As the air enters the collection chamber 70it is directed through the orifices 63 into the diffuser chamber 42since it is at a higher pressure than the air in that chamber. It hasbeen found that the injection of air into the diffuser will greatlydecrease the effects of surging. Known means and techniques may beutilized to control valve 61 as a function of the outlet flow from thecompressor. For example, a Pitot tube may be inserted within outlet 62and utilized to operate valve 61.

While in FIGS. 2 through 4 the chamber 70 and orifices 63 areillustrated disposed along an inner radius of the diffuser manifold, itis of course within the scope of the present invention to place suchchamber and orifices along the side of the diffuser or at any convenientlocation. Additionally, while the presently preferred embodiment hasbeen illustrated as a two-stage compressor having stages of compression40 and 50, it is within the scope of the present invention to utilizethe presently disclosed surge control means with a compressor having anynumber of stages of compression. In such cases the air from any stage ofcompression except the first stage may be injected into the diffuser ofany lower state of compression, as taught by the present invention.Also, in some applications it may be desirable to use the surge controlmeans illustrated in FIGS. 2 through 4 in combination with some priorart surge control means such as the one illustrated in FIG. 1.

Thus, a surge control means for use with a centrifugal compressor hasbeen disclosed wherein air from a higher stage of compression isinjected into the diffuser of a lower stage of compression therebypreventing surging.

I claim:

1. In a centrifugal fluid compressor such as an air compressor having aplurality of stages of compression at least one of which includes adiffuser, surge control means for controlling surging in the compressorcomprising:

a manifold coupled to said diffuser of one stage of compression defininga volume which communicates with said diffuser through orifices, saidmanifold including an inlet such that inlet fluid to said inlet enterssaid diffuser through said orifice;

a line coupled between said inlet of said manifold and a higher stage ofcompression than said one stage of compression such that fluid may beremoved from said higher state of compression and injected into thediffuser of said one stage of compression.

2. The means for controlling surging defined in claim 1 wherein acontrol means, for controlling the flow of fluid in said line, iscoupled to said line.

3. The means for controlling surging defined in claim 1 wherein saidorifices are disposed such that fluid entering said diffuser from saidvolume is injected in the direction of the prevailing flow direction insaid diffuser.

4. The means for controlling surging defined in claim 3 wherein saidorifices, are tangentially disposed through said diffuser.

5. In a centrifugal air compressor which includes a plurality of stagesof compression, each stage comprising a rotor and diffuser, surgecontrol means comprisa manifold defining a collection chamber disposedabout the diffuser of one stage of compression said manifold having aninlet, and said collection chamber communicating with said diffuserthrough a plurality of orifices;

a line for transporting air, coupled between said manifold inlet and astage of compression higher than said one stage of compression such thatair from said higher stage of compression may be injected into thediffuser of said one stage of compression, and

control means for controlling the flow of air in said line as a functionof the output flow of air from said compressor.

6. The surge control means defined in claim 5 wherein said orifices aredisposed such that fluid entering said diffuser from said collectionchamber is injected in the direction of the prevailing flow in saiddiffuser.

7. The surge control means defined in claim 6 wherein said orifices aretangentially disposed into said diffuser.

8. The surge control means defined in claim 7 wherein said compressorhas two stages of compression.

9. The surge control means defined in claim 6 wherein said control meansincludes a valve disposed in said line.

1. In a centrifugal fluid compressor such as an air compressor having aplurality of stages of compression at least one of which includes adiffuser, surge control means for controlling surging in the compressorcomprising: a manifold coupled to said diffuser of one stage ofcompression defining a volume which communicates with said diffuserthrough orifices, said manifold including an inlet such that inlet fluidto said inlet enters said diffuser through said orifice; a line coupledbetween said inlet of said manifold and a higher stage of compressionthan said one stage of compression such that fluid may be removed fromsaid higher state of compression and injected into the diffuser of saidone stage of compression.
 2. The means for controlling surging definedin claim 1 wherein a control means, for controlling the flow of fluid insaid line, is coupled to said line.
 3. The means for controlling surgingdefined in claim 1 wherein said orifices are disposed such that fluidentering said diffuser from said volume is injected in the direction ofthe prevailing flow direction in said diffuser.
 4. The means forcontrolling surging defined in claim 3 wherein said orifices aretangentially disposed thrOugh said diffuser.
 5. In a centrifugal aircompressor which includes a plurality of stages of compression, eachstage comprising a rotor and diffuser, surge control means comprising: amanifold defining a collection chamber disposed about the diffuser ofone stage of compression said manifold having an inlet, and saidcollection chamber communicating with said diffuser through a pluralityof orifices; a line for transporting air, coupled between said manifoldinlet and a stage of compression higher than said one stage ofcompression such that air from said higher stage of compression may beinjected into the diffuser of said one stage of compression, and controlmeans for controlling the flow of air in said line as a function of theoutput flow of air from said compressor.
 6. The surge control meansdefined in claim 5 wherein said orifices are disposed such that fluidentering said diffuser from said collection chamber is injected in thedirection of the prevailing flow in said diffuser.
 7. The surge controlmeans defined in claim 6 wherein said orifices are tangentially disposedinto said diffuser.
 8. The surge control means defined in claim 7wherein said compressor has two stages of compression.
 9. The surgecontrol means defined in claim 6 wherein said control means includes avalve disposed in said line.